“Transmission format” means a set of parameters determining the formatting of data transmitted over a radio-frequency propagation channel, referred to as the “channel”, between a transmitting device and a receiving device of a satellite telecommunications system, for example a channel between a satellite and a terrestrial terminal.
Said parameters are generally a modulation type and a channel coding rate; each transmission format generally corresponds to a specific data rate.
As an example, the specifications of the DVB-S2 (“Digital Video Broadcast—Satellite—Second Generation”) standard provide for four different modulation types (QPSK, 8PSK, 16APSK and 32APSK) and eleven different coding rates (from ¼ to 9/10), generating twenty-eight different transmission formats.
The advantage of having several possible transmission formats lies in the possibility of adapting the transmission format to the instantaneous channel quality of each terrestrial terminal. “Instantaneous channel quality” means the channel quality as perceived at the scale of the duration of a data packet (in the millisecond range in the case of a DVB-S2 satellite telecommunications system).
A terrestrial terminal that has a good quality channel may be assigned a transmission format corresponding to a high data rate, whereas a terrestrial terminal that has a poor quality channel will be assigned a transmission format corresponding to a lower data rate.
Generally, a transmission format is selected by comparing the estimated value of the instantaneous channel quality to predefined thresholds, representing theoretical values of the instantaneous channel quality required for using different transmission formats.
The thresholds are static, fixed beforehand by the telecommunications system operator, and determined so as to theoretically ensure, for each transmission format, that an error rate in the data received is below a maximum tolerated error rate. In addition, it is common to provide margins between the selected thresholds and the theoretical values of the instantaneous quality of the propagation channel for which the maximum error rates are obtained, so as to protect against any inaccuracies in the estimated value of the instantaneous channel quality, related to the inaccuracy of the estimator considered, variations in the channel, etc.
The utilization of such static thresholds presents drawbacks.
Firstly, the static thresholds are the same for all terrestrial terminals, and do not take into account the special features of the various channels between the satellite and each terrestrial terminal. As a result significant margins are introduced, so that the bandwidth between the satellite and the terrestrial terminals is largely under-used.
In addition, the static thresholds do not allow the changes over time in the various channels to be taken into account. In particular, the time period, referred to as the “loop time”, between the point in time when the instantaneous channel quality is estimated and the point in time when the transmission format, selected on the basis of said estimated value, will actually be applied and presented to the receiver, can vary in practice between 0.5 seconds and 5 seconds in a satellite telecommunications system such as DVB-S2, where the satellites are stationed in a geostationary orbit (GEO). It should be noted that the value of 0.5 seconds corresponds to an absolute minimum value for the loop time, induced by the propagation times for signals between the Earth and a satellite in a GEO orbit.
Variations in the instantaneous channel quality can occur during the loop time, and the selected transmission format may no longer be suitable. The variations in the channel are all the more significant when high frequency bands are used, e.g. Ka, Q, V, etc. bands.
In the context of satellite telecommunications system, it is known from European patent application EP 2061167 to periodically transmit a learning sequence for each available transmission format, check which learning sequences have been correctly demodulated/decoded and select, for transmitting user data packets, a transmission format for which the corresponding learning sequence has been correctly demodulated/decoded.
This method has the disadvantage of reducing the bandwidth available for transmitting user data, because of the need to periodically transmit learning sequences for each transmission format available (a learning sequence corresponding to data known beforehand by the satellite and the terrestrial terminal). In addition, this method does not take into account possible variations in the instantaneous channel quality, related in particular to the loop time needed to apply the selected transmission format.